Import various libgcc functions from syslinux.

Experimentation reveals that gcc ignores -mrtd for the implicit
arithmetic functions (e.g. __udivdi3), but not for the implicit
memcpy() and memset() functions.  Mark the implicit arithmetic
functions with __attribute__((cdecl)) to compensate for this.

(Note: we cannot mark with with __cdecl, because we define __cdecl to
incorporate regparm(0) as well.)

src/Makefile

@@ -145,6 +145,7 @@ DEBUG_TARGETS	+= dbg%.o c s
 
 # SRCDIRS lists all directories containing source files.
 #
+SRCDIRS		+= libgcc
 SRCDIRS		+= core
 SRCDIRS		+= proto
 SRCDIRS		+= net net/tcp net/udp